1. Field of the Invention
This invention relates to a magnetic field generating device employing permanent magnets and used for nuclear magnetic resonance-computerized tomography (hereinafter referred to as NMR-CT) which obtains a sectional image of an object subjected to medical examination and picturizes up to the property of tissue and, particularly, to the magnetic field generating device of generating a strong, accurate, uniform magnetic field within a sufficiently large air gap space.
2. Description of the Prior Art
In order to obtain a desired tomographic image of the human body by inserting the whole body on its portion in an air gap where a strong magnetic field on the order of 1-10 kG (0.1-1 T) is created, a uniformity and stability on the order of 10.sup.-4 or less are also required for the NMR-CT usage.
Recently, as the magnetic field generating device for NMR-CT, one type of device employing permanent magnets as the magnetic field generating source has attracted attention because it causes no consumption of huge electric power and of coolant such as helium and has a configuration easy for maintenance as compared with the other types employing resistive electro magnets or superconductive magnets. Previously, the present applicant proposed one magnetic field generating device of high practicability which includes permanent magnets of superior magnetic characteristic arranged effectively therein (see U.S. Pat. Ser. No. 719,820 or E.P. No. 0161782 Al).
Specifically, as shown in FIG. 8, this proposed magnetic field generating device is characterized in that permanent magnets 1, 1 of a pair magnetized in the same direction are spaced from each other to define an air gap 4 therebetween with their different-polarity poles opposing mutually, to the opposing surface of each permanent magnet is attached a magnetic pole segment 2 having an annular projection 5 or a magnetic pole segment 2 having an annular projection 5 and a convex projection 6 formed in the central portion of the annular projection 5, and these permanent magnets are magnetically coupled together by a yoke 3, thereby creating a magnetic field within the air gap 4 (the magnetic pole segment illustrated has both the annular projection and the convex projection formed thereon).
The aforementioned magnetic field generating device has improved remarkably the uniformity of the magnetic field created within the air gap. However, this magnetic field generating device tends to cause leakage of magnetic flux from the magnetic pole segment attached to one end of each permanent magnet to the outside of the air gap and has the characteristic that on the vertical center line of the air gap, the closer to the magnetic pole surface, the stronger is the magnetic field. Therefore, to get a required high degree of uniformity of the magnetic field within the serviceable magnetic field space, it is necessary to enlarge the air gap more than a few times the serviceable magnetic field space by increasing, for example, the inter-pole distance or the area of the magnetic pole, hence, this measure limits miniaturization of the device. Accordingly, it is demanded to improve further the aforementoned magnetic field generating device to enhance its practicability, and especially, a novel configuration is strongly demanded which can concentrate the magnetic flux originating from the permanent magnet effectively within the air gap to thereby permit volume reduction, miniaturization, and weight reduction of the permanent magnet.